1. Field of the Invention
The present invention relates to a liquid crystal display element for displaying images by driving a liquid crystal, a driving method of the element, and an electronic paper having the element.
2. Description of the Related Art
Recently, the development of an electronic paper is active in various enterprises and universities. Promising fields of application of the electronic paper include the field of an electronic books first of all and include the field of portable apparatus such as sub-displays of mobile terminals and IC card display units or the like. One type of display elements used in an electronic paper is liquid crystal display elements using a liquid crystal composition in which a cholesteric phase is formed (such a composition is called “a cholesteric liquid crystal” or “a chiral nematic liquid crystal” and hereinafter referred to as “a cholesteric liquid crystal”). A cholesteric liquid crystal has excellent characteristics such as semi-permanent display content holding properties (memory characteristics), vivid color display, high contrast, and high resolution.    Patent Document 1: JP-A-2001-228459    Patent Document 2: JP-A-2003-228045    Patent Document 3: JP-A-2000-2869    Patent Document 4: JP-A-2000-147466    Patent Document 5: JP-A-2000-171837    Patent Document 6: International Patent Publication No. 06/103,738 Pamphlet    Non-Patent Document 1: Nam-Seok Lee, Hyun-Soo Shin, etc., A novel Dynamic Drive Scheme for Reflective Cholesteric Displays, SID 02 DIGEST, pp. 546-549, 2002    Non-Patent Document 2: Y.-M. Zhu, D.-K. Yang, Cumulative Drive Schemes for Bistable Reflective Cholesteric LCDs, SID 98 DIGEST, pp. 798-801, 1998
A description will now be made on the documents on the related art disclosing methods of multi-grayscale display utilizing cholesteric liquid crystals and problems of those methods.
For example, Patent Documents 1 and 2 disclose methods called dynamic driving in which intermediate grayscales are displayed using amplitudes, pulse widths, or phase differences in a selection section among three sections of a driving waveform, i.e., a preparation section, a selection section, and an evolution section. Although such dynamic driving methods allow driving at a high speed, a problem arises in that intermediate grayscales have high granularity.
In general, dynamic driving requires dedicated driving devices (drivers) to allow a multiplicity of voltages to be output, and a cost increase can result from the fabrication of the drivers and the complicatedness of a driver control circuit.
Non-Patent Document 1 discloses a dynamic driving method which is implemented using inexpensive general-purpose STN drivers. However, the elimination of high granularity constituting a problem of dynamic driving cannot be expected from the method.
Patent Document 3 discloses a method having the steps of applying a first pulse to a liquid crystal to put it in a homeotropic state and applying second and third pulses immediately after the first pulse to display a desired grayscale using a potential difference between the second and third pulses. According to this driving method, the concern about the granularity of intermediate grayscales remains, and another problem arises in that an element cannot be manufactured with an inexpensive configuration because a high driving voltage is required.
All of the above-described driving methods according to the related art are driving methods utilizing an intermediate grayscale region B as shown in FIG. 4 which will be described later. Therefore, the methods have a problem with display quality because of significant granularity of images obtained thereby, although they allow driving at a high speed. A driving method utilizing an intermediate grayscale region A as shown in FIG. 4 is disclosed in Non-Patent Document 2, and the method still has a problem.
Non-Patent Document 2 discloses a method which utilizes cumulative response (overwrite) characteristics unique to liquid crystals to drive a liquid crystal from the planar state to the focal conic state or from the focal conic state to the planar state gradually at a high speed on the order of the rate of quasi moving pictures by applying relatively short pulses to the liquid crystal.
However, this method requires a driving voltage as high as 50 to 70 V to perform driving at such a relatively high speed, and it can therefore result in a cost increase. Further, the method which is referred to as “two phase cumulative drive scheme” involves two stages, i.e., “a preparation phase” and “a selection phase”. Since responses in two directions, i.e., cumulative responses toward the planar state and cumulative responses toward the focal conic state (the intermediate grayscale region A and the intermediate grayscale region B) are used at those phases, respectively, a problem in display quality arises.
Patent Documents 4 and 5 disclose methods including the use of a fast forward mode which takes advantage of resetting to the focal conic state. Although such a method provides relatively high contrast compared to the above-described methods, writing after a reset requires a high voltage which is difficult to supply using general-purpose STN drivers. Further, such a method has a problem in that it results in increased cross-talks to half-selected or non-selected pixels because grayscales are written in a cumulative manner during the focal conic state to the planar state.
Patent Document 6 discloses a method which takes advantage of cumulative responses (overwrites) of a liquid crystal to achieve multi-grayscale display having high uniformity with a liquid crystal display element using inexpensive general-purpose drivers having a low breakdown voltage. According to this method, a voltage pulse is applied to a liquid crystal layer a plurality of times to vary a driving voltage and a pulse width stepwise, whereby the liquid crystal is controlled to change from an initial state that is a reflective state to a predetermined intermediate grayscale state using a region having a great margin (intermediate grayscale region A). Since any increase in the driving voltage can be consequently avoided, the method can be implemented using inexpensive general purpose drivers which have a low breakdown voltage and which provide binary outputs. Further, since this method allows gray level conversion utilizing a region having a great margin, multi-grayscale display can be achieved with high uniformity. However, this method has following problems.
A first problem is that blurs and ghosts can occur on a display screen. Since a reset voltage of a resetting unit used in this method depends on image data to be displayed, the resetting effect varies from pixel to pixel. As a result, displayed characters may be blurred, and ghosts may appear.
A second problem is that a grayscale jump can occur when low grayscales are displayed. According to this display method, there is a great difference in brightness between the lowest grayscale (black) and the grayscale one level higher than the lowest grayscale, and a problem arises in that grayscale jumps are noticeable when the low grayscales are displayed. According to this method, white and black are written at the first scan, and intermediate grayscales are written at the subsequent scans by applying short pulses in a cumulative manner. However, cumulative response is degraded for low grayscales. This results in a great difference in brightness between the lowest grayscale and the grayscale one level higher than the lowest grayscale which is written by applying a voltage pulse of ±20 V to the liquid crystal layer.
A third problem is an increase in rewriting time. The display method according to the related art requires a resetting time of 5.4 seconds and a grayscale writing time of 6.9 seconds, for example, in the case of a screen having XGA resolution. Therefore, displayed content cannot be recognized for at least 5.4 seconds until resetting is completed. Under the circumstance, there are demands for a novel display method which enables display within about 2 seconds even if there is some reduction in contrast.